In order to control the bonding layer thickness between flat panel displays, such as LCD's, microspheres have been used to set the distance between two substrates. This is important for achieving the electrical requirements of flat panels, such as contrast, speed of display, etc. Optical disc manufacturing requires similar control of bonding layer thickness, although for different reasons. The method of using microspheres has been modified as set forth herein and advantageously utilized in the manufacturing of optical discs, in order to improve the bonding layer uniformity.
Some prior solutions have used the center hole burr from the punch and die process in the optical disc manufacturing process as being about the correct thickness and have relied on this thickness. In another previous attempted solutions, a calculated mass has been used to apply pressure to the two discs as they are being bonded to obtain the correct thickness. However, these prior solutions to control the bonding layer thickness between optical discs have been found to be disadvantageous in that (1) the thickness of the bonding layer is dependent upon other processes, (2) the thickness of the bonding layer does not tend to remain consistent across the entire disc, and (3) the thickness of the bonding layer may vary across the disc with respect to the pressure applied thereto.
Moreover, in manufacturing DVD-9 optical discs, any method to control the bonding adhesive thickness must meet its lateral and parallelism requirements. A schematic representation of the lateral requirements of a DVD-9 optical disc is shown in FIG. 1. This specification for overall bonding layer thickness is set at 55.+-.15 .mu.m and is required because the player must be able to focus on both the top and bottom discs. In addition, the focus servo has to have the ability to move within this specification. It has been found that a thickness greater than this specification can cause errors in reading information on the disc.
The parallelism requirements of a DVD-9 disc are schematically illustrated in FIG. 2. In a DVD-9 disc, the specification for variation of the thickness of any one disc is .+-.10 .mu.m. In addition, the specification for thickness differences within one revolution is .+-.4 .mu.m. These specifications are required because the focus servo will not be able to respond quickly enough to compensate for greater changes. If the focus servo is not so able to respond, errors in reading information on the disc result. Furthermore, if the disc is not parallel, it will be unbalanced. As technology calls for faster players, such as 24x or 32x, this unbalance will cause the disc to wobble thereby contributing to even lower performance.